Pulps produced by either mechanical or chemical pulping methods possess a color that can range from dark brown to creamish depending on the wood type and defibering process used. The pulp is bleached to produce white paper products for a multiplicity of applications.
Bleaching is the removal or alteration of those light-absorbing substances found in unbleached pulp. In the bleaching of mechanical pulp, the object is to decolorize the pulp without solubilizing the lignin. Either reducing (e.g. sodium hydrosulfite) or oxidizing (e.g., hydrogen peroxide) bleaching agents are usually used. The bleaching is often a multistage process. The bleaching of chemical pulps is an extension of the delignification that started in the digestion stage. The bleaching is often a multistage process, which stages may include chlorine dioxide bleaching, oxygen-alkaline delignification, and peroxide bleaching.
Discoloration, mostly ascribed to thermal aging, results in yellowing and brightness loss in various stages of papermaking processes employing bleached pulp and in the resultant paper products. The industry invests significantly in chemicals such as bleaching agents and optical brighteners that improve optical properties of the finished paper or paper products.
For example, post bleaching processes in which pulp is brought to the desired whiteness using peracids acid are disclosed in WO 0052258 and WO 9932710. Addition of a composition comprising peracetic acid and hydrogen peroxide to thick stock prior to the addition of optical brightening agents (OBA's) is claimed to reduce the amount of OBA's required to achieve a comparable degree of brightness in GB2391011. Jukka Jakara et al., The effect of peracetic acid treatment of bleached kraft pulp in fine paper production, Preprint-PAPTAC Annual Meeting, 87th, Montreal, QC, Canada Jan. 30-Feb. 1, 2001 (2001) discloses that addition of peracetic acid to the stock preparation chests of paper machines limits brightness reversion in bleached pulp and results in significant savings in OBA addition in the paper machine. See also, Jukka Jakara et al., The effect of peracetic acid in fine paper production, Appita Annual Conference Proceedings (2000), 54th (Vol. 1), 169-174 and Jukka Jakara et al., The use of peracetic acid as a brightening agent, Appita Ann. General Conf. Proc. (1999), 53rd (Vol. 2), 463-467. Treatment of bleached stock with peroxyacid prior to addition of neutral size to the stock is disclosed in FI104339B. Peroxide preparations containing optical brighteners in encapsulated form are disclosed in CA2292107.
To date, however, the results have been less than satisfactory and the economic losses resulting from discoloration and yellowing present significant ongoing challenges to the industry. Accordingly, there remains a need for a successful and practical solution to loss of brightness and undesirable yellowing of pulp and paper.